This invention relates to the art of developing resist patterns through removal of parts of a resist layer on an electroconductive surface with use of a developing liquid. More specifically, the invention deals with a method of, and an apparatus for, automatically precisely developing resist patterns on substrates such as bodies of vitreous material with an electroconductive layer thereon.
The lithographic process has been widely practiced for the fabrication of photomasks and conductive patterns on semiconductor wafers. The process is such that a resist coating on a desired substrate is first subjected to a patternmaking process such as selective exposure to electromagnetic rays such as ultraviolet rays, or drawing with a beam of particles such as electrons. Then the resist coating is developed into a desired pattern with use of a chemical developing liquid. The development of the resist coating is possible by either dipping, spraying, or paddling.
Japanese Unexamined Patent Publication No. 62-117324 proposes a developing apparatus based on the dip method. This prior art apparatus comprises an open-top developing vessel, which may be either fixed or movable, to be filled with a developing liquid. The developing vessel has disposed therein a fixed or movable table on which is to be placed a workpiece having a resist coating that has undergone the process of exposure or drawing. Dipped in the developing liquid within the vessel, the resist coating can be automatically developed into a desired pattern with high precision.
One of the features of the prior art apparatus is the provisions for recirculating and reconditioning the used developing liquid. As each workpiece is processed with a fresh developing liquid, the production of defects in the developed resist patterns is reduced to a minimum.
However, the above cited prior art apparatus has proved to have a serious drawback. The drawback may arise in the event of a change in the conditions of the preceding process of exposure or drawing, in the temperature or kind of the developing liquid in use, or in the physical conditions of the resist coatings. In such event the length of time needed for developing each workpiece may become shorter than that for the recovery of the used developing liquid from within the vessel. Then the prior art apparatus becomes totally incapable of use. The following is a more detailed discussion of how this inconvenience takes place.
In the production of photomasks, as heretofore practiced in the art, the resist is coated on an electroconductive layer, such as chromium, that has been preformed on a substrate of vitreous material. With this workpiece dipped in a bath of developing liquid, a voltage is impressed between the electroconductive layer and an electrode also immersed in the bath. The current flowing as a result of the voltage application varies in magnitude with the progress of the etching of the resist coating (see FIG. 5). The current magnitude first decreases with the progress of development, then starts increasing as the electroconductive layer becomes partly exposed, and then again decreases after reaching a peak value.
A main developing operation continues a time tm until the current magnitude reaches the peak value. Then the workpiece is subjected to additional development for a time equal to 20 percent of the main developing time tm. With the resist pattern developed in this manner, the opening in the pattern that is designed to be 2.00 micrometers in width will have an actual width of 2.03 micrometers or so.
If the main developing time tm is 288 seconds, the additional developing time is approximately 58 seconds (288.times.0.2). The prior art apparatus takes approximately 30 seconds for recovering 95 percent of the developing liquid that has been used for processing each workpiece. Being laid horizontally and covered with the remaining liquid, the workpiece is still undergoing the process of development during this 30-second recovery time. Consequently, the additional developing time must actually be 28 seconds (58-30).
Let it be supposed that the main developing time tm is cut down to, say, 134 seconds, due for example to a higher degree of exposure to which the resist coating has been subjected, or to some other change in the processing conditions or in the physical conditions of the resist coating itself. Then the additional developing time is also reduced to 27 seconds (134.times.0.2), which is shorter than the recovery time. Since development continues during the recovery time as aforesaid, the precise creation of resist patterns becomes impossible. For example, the resist pattern opening that is designed to be 2.00 micrometers in width may actually become as wide as 2.10 micrometers.
Therefore, conventionally, the recovery time has had to be cut short for the creation of resist patterns with a desired degree of precision. A substantial proportion of the used developing liquid has been drained and wasted.